Some embodiments described herein relate generally to methods and apparatus for the blind time skew detection and compensation for coherent optical receivers.
Known coherent optical receivers typically use analog-to-digital converters (ADCs) to digitally sample the received signal. After digitization (due of the coherent detection of the optical electric field), the signal can be processed and optical and/or electrical impairments can be compensated using various digital signal processing (DSP) techniques. Before digitization, the signal coherently beats with a reference local oscillator via a down converter (or optical hybrid) that can introduce time skews between the in-phase (I-tributary) and the quadrature (Q-tributary) of the coherent optical receiver.
Typically, one ADC is used for the real (in phase) signal component and another ADC is used for the imaginary signal component for each polarization. The two ADCs may not have a mechanism to synchronize with each other after being powered up. This can result in an unpredictably large time skew in the range of several hundreds of symbols. If the time skew between the I-tributary and the Q-tributary is greater than a threshold (e.g., 10% of the symbol duration), an unacceptable performance penalty occurs. For example, at 32 Gbaud, a time skew greater than 3 ps produces a non-negligible penalty.
Accordingly, a need exists for methods and apparatus for time skew detection and compensation of coherent optical receivers.